Research Byte
Published in the RSAA Lunations
Vol1 Issue11 1–31 December 2020
Emu: Near-infrared sky survey of cool stars from the International Space Station
Our galaxy abounds with M-dwarf stars, however due to their intrinsic faintness and very cool temperatures, relatively little is known about their make-up – i.e. their abundance patterns of elements heavier than hydrogen and helium. Their low temperatures amounts to the outer atmospheres showing a complex soup of atomic and molecular absorption lines, making meaningful measurement of key elemental abundance patterns challenging. Oxygen is an excellent tracer of the underlying abundance or metal composition of cool stars. A near-infrared sky survey in the 1.4 µm ‘water band’ will provide the oxygen abundance of cool-stars to deliver critical stellar properties of these systems.
The Emu survey employs the high-speed time-delay integration (TDI) technique to scan the sky at 1.4 µm without active pointing control. Emu is a 6U (size of a shoe box) form factor space payload to be hosted on the International Space Station (ISS). It will provide powerful new observations at the critical water absorption wavelength inaccessible to ground-based telescopes due to the Earth’s own atmosphere. Conventional infrared detectors would be unsuitable to satisfy the sensitivity requirements of the Emu mission due to their readout noise. Hence, Emu employs the paradigm-changing properties of the Leonardo 'noise-free' SAPHIRA linear-mode avalanche photodiode array. This detector provides a significant signal gain before readout and the array will be read at 50 Hz via Rosella control electronics. A compact linear Stirling cryo-cooler will provide cooling to around 80 K for the focal plane array.
The near-infrared market for remote sensing Earth observations is large, with important applications in agricultural monitoring, mineralogical survey analysis as well as defence applications. The challenge is to advance low-cost commercial systems, such as that at the heart of this mission, beyond low level laboratory development to demonstrate flight heritage. Undertaking a rigorous scientific program, of modest and self-contained scope but with wide reach across contemporary astronomy, will properly exercise the observing system and allow full power of the SAPHIRA detector to be demonstrated in context. The ability to operate at high full-frame rates while free from electronic noise fundamentally changes the way in which near-infrared imaging (and spectroscopy/hyperspectral) is performed for astronomy and remote sensing earth observations. The Emu W-band survey mission will be the first science program to demonstrate the power of these new devices for astronomy and science in general.
Joice Mathew
Fig: Emu Water band and synthetic spectra of two cool stars, (right) Emu instrument layout
